1. Technical Field
The present invention relates to a heat exchanger, and more particularly, to a heat exchanging condenser for use in an automotive air-conditioning system.
2. Description of the Prior Art
With reference to FIG. 1, a conventional refrigeration circuit for use, for example, in an automotive air-conditioning system is shown. Circuit 1 includes compressor 10, condenser 20, receiver or accumulator 30, expansion device 40, and evaporator 50 serially connected through pipe members 60 which link the outlet of one component with the inlet of a successive component. The outlet of evaporator 50 is linked to the inlet of compressor 10 through pipe member 60 so as to complete the circuit. The links of pipe members 60 to each component of circuit 1 are made such that the circuit is hermetically sealed.
In operation of circuit 1, refrigerant gas is drawn from the outlet of evaporator 50 and flows through the inlet of compressor 10, and is compressed and discharged to condenser 20. The compressed refrigerant gas in condenser 20 radiates heat to an external fluid flowing through condenser 20, for example, atmospheric air, and condenses to the liquid state. The liquid refrigerant flows to receiver 30 and is accumulated therein. The refrigerant in receiver 30 flows to expansion device 40, for example, a thermostatic expansion valve, where the pressure of the liquid refrigerant is reduced. The reduced pressure liquid refrigerant flows through evaporator 50, and is vaporized by absorbing heat from a fluid flowing through the evaporator, for example, atmospheric air. The gaseous refrigerant then flows from evaporator 50 back to the inlet of compressor 10 for further compressing and recirculation through circuit 1.
With further reference to FIGS. 2 and 2a, conventional heat exchanging condenser 20 is shown. Condenser 20 includes a plurality of adjacent, essentially flat tubes 21 having oval cross section and open ends which allow refrigerant fluid to flow therethrough. Flat tubes 21 may include a plurality of parallel passages. A plurality of corrugated fin units 22 are disposed between adjacent tubes 21. Header pipes 23 and 24 are disposed perpendicularly to flat tubes 21, at each open end. Inlet tube 31 and outlet tube 32 are connected to header pipes 23 and 24 and allow condenser 20 to be linked to the other elements of the circuit by pipe member 60 as shown in FIG. 1.
With further reference to FIG. 3, each header pipe 23 and 24 may have a clad construction and include central tube 26 which may be made from aluminum, and inner and outer metallic tubes or layers 27 and 28 which are brazed to the inner and outer surfaces of central tube 26, respectively. Central tube 26 includes slots 29 disposed therethrough. Flat tubes 21 are fixedly connected to header pipes 23 and 24 and are disposed through slots 29 such that the open ends of flat tubes 21 communicate with the hollow interiors of header pipes 23 and 24. Inner and outer tubes 27 and 28 include brazing portions 27a and 28a which define openings corresponding to slots 29 in central tube 26. Flat tubes 21 are inserted in slots 29, and portions 27a and 28a are brazed to the exterior surface of flat tubes 21 near the open ends to ensure that flat tubes 21 are fixedly and hermetically sealed within header pipes 23 and 24.
In operation, compressed refrigerant gas from compressor 10 flows into first header pipe 23 through inlet pipe 31, and is distributed such that a portion of the gas flows through each of flat tubes 21 and into second header pipe 24. As the refrigerant gas flows through flat tubes 21, heat from the refrigerant gas is exchanged with the atmospheric air flowing through corrugated fin units 22 in the direction of arrow W as shown in FIG. 2a. Since the refrigerant gas radiates heat to the outside air, it condenses to a liquid mist as it travels through tubes 21. The liquid mist is collected in second header pipe 24, and flows out therefrom through outlet pipe 32 and into receiver 30 where the mist accumulates, and then to the further elements of the circuit as discussed above.
Flat tubes 21, which are generally made of aluminum or an aluminum alloy which comprises substantially aluminum, are subjected to corrosion during normal operation of condenser 20. For example, flat tubes 21 may undergo pitting at many locations on the surface thereof. The pits may eventually develop into openings formed through the surfaces of flat tubes 21, allowing leakage of the refrigerant fluid from condenser 20. Several methods of improving the corrosion resistance of flat tubes 21 are known in the prior art. A first method of improving the corrosion resistance of flat tubes 21 is accomplished by increasing the difference in potential between the materials which make up the flat tubes and the materials which make up the corrugated fin units. That is, the flat tubes are made of materials with a higher potential than the material from which the fin units are made.
The increase in the potential difference may be accomplished by one of two techniques. As an example only, flat tubes 21 may be made of aluminum alloy AA1070, which comprises by weight 0.20% or less Si, 0.25% or less Fe, 0.04% or less Cu, 0.03% Mn, 0.03% or less Mg, 0.04% or less Zn 0.05% or less V, 0.03% or less Ti and the balance substantially aluminum. As shown in FIG. 9, fin units 22 may include core portion 221 comprising AA3003 which comprises by weight 0.6% or less Si, 0.7% or less Fe, 0.05-0.20% Cu, 1.0-1.5% Mn, 0.10% or less Zn, and the balance substantially Al, and inner and outer surface portions 222 and 223 made of AA4045 which comprises by weight, 0.30% or less Cu, 5-13% Si, 0.8% or less Fe, 0.15% or less Mn, 0.1% or less Mg, 0.20% or less Zn, 0.20% or less Ti, and the balance substantially Al. In the first technique, the material from which the corrugated fin units are constructed would be selected so as to decrease the potential as compared to the potential of the material from which flat tubes 21 are constructed. With respect to the present example, the first technique may be accomplished by constructing corrugated fin units 22 out of an aluminum alloy with an increased zinc content, for example, portions 222 and 223 will be made of AA4045 with an additional 1.0% zinc added thereto. Since, corrugated fin units 22 will have an increased proportion of zinc, they will also have a decreased potential as well. Therefore the potential difference between flat tubes 21 and fin units 22 is increased, reducing pitting of the flat tubes.
In the second technique, the material from which flat tubes 21 are constructed would be selected so as to increase the potential as compared to the potential of the material from which fin units 22 are constructed. In the present example, the second technique may be accomplished by constructing flat tubes 21 out of an aluminum alloy with an increased copper content, for example, AA1070 having an increased copper content of 0.35-0.65%. Since flat tubes 21 will have an increased proportion of copper, they will also have an increased potential as well. Therefore the potential difference between flat tubes 21 and fin units 22 is again increased, reducing pitting of the flat tubes.
Although both of the techniques of the above method for constructing the heat exchanger result in a condenser in which flat tubes 21 have increased resistance to corrosion, flat tubes 21 are still prone to undergoing pitting. Eventually, the pitting of flat tubes 21 will result in openings forming through the surfaces of flat tubes 21 and allowing undesirable leakage of the refrigerant from condenser 20 to the outside environment.
A second method of improving the corrosion resistance of flat tubes 21 is accomplished by treating the surfaces of flat tubes 21 such that they are more resistant to pitting. Once again, assuming as an example only that flat tubes 21 are made of AA1070 and fin units 22 are made of a core of AA3003 and outer surface portions of AA4045, flat tubes 21 may be treated according to two techniques. The first technique comprises a galvanizing process in which flat tubes 21 are dipped in a bath of zinc oxide (ZnO) and sodium hydroxide (NaOH). The zinc is diffused through flat tubes 21 due to a displacement reaction. The galvanized flat tubes have increased resistance to pitting. After galvanizing, the overall composition of flat tubes 21 will include 3.0-4.0% Zn. This method is known as disclosed in Japanese Patent Application laid open Gazette No. 56-155,398.
The second technique for treating flat tubes 21 is by zinc spraying. In this technique, zinc wire or powder is fed at a controlled rate into the flame of an oxygas or oxyacetylene torch. The zinc is atomized, and impinges on the external surfaces of the flat tubes to produce a layer of flattened and interlocked particles which are mechanically bonded to the surface being coated. As with galvanizing, the overall composition of flat tubes 21 will include 3.0-4.0% Zn. Once again, this process offers increased resistance to pitting.
However, even though both galvanizing and zinc spraying offer increased resistance to pitting, the flat tubes treated in this manner are more likely to undergo corrosion due to stratiform corrosion, that is, corrosion which occurs in even layers, than non-treated flat tubes. Since stratiform corrosion is a slower process in which a whole layer of the flat tube corrodes simultaneously, it takes longer for openings to form through the surface of flat tubes 21 than when flat tubes 21 are more susceptible to pitting as in the first method. Thus, by using the second method, the usable lifetime of flat tubes 21 is increased as compared to the situation in which the flat tube is untreated or is treated by the first method. In practice, since better overall corrosion resistance is provided by the second method in which the form of corrosion that the flat tubes are likely to undergo is changed from pitting to stratiforming, this method is preferred and used more frequently than the first method.
However, even in a heat exchanger in which the second method is utilized and pitting is prevented, undesirable leakage of refrigerant fluid from the condenser still occurs. Specifically, as shown in FIG. 4, in the second prior art method, the entire exterior surfaces of flat tubes 21 are covered by zinc layer 210. Therefore, zinc layer 210 extends through slots 29 of header pipes 23 and 24 such that brazing portions 27a and 28a are brazed to the header pipes at zinc layers 210. The left side of the figure shows a flat tube including a zinc layer which has not undergone stratiform corrosion, and thus, the condenser is effectively hermetically sealed at the location where brazing portions 27a and 28a are brazed to flat tubes 21. However, as shown in the right side of the figure, after continued operation of condenser 20, the stratiform corrosion of the surface of flat tubes 21 will extend into the area where portions 27a and 28a are brazed to flat tubes 21, thereby decreasing the effective hermetic sealing of the condenser. As shown in the figure in exaggerated detail for clarity, stratiforming of flat tubes 21 results in gaps G forming between the surfaces of tubes 21 and brazing portions 27a and 28a and central tube 26. These gaps allows the refrigerant fluid to leak from header pipes 23 and 24 to the exterior region of heat exchanger 20.